162 



METHODS OF COMMUNICATION FOR FOREST PROTECTION 



apart, their height, and the height of the intervening ground. The curvature of the 

 earth is the important controlling factor, although signals may be exchanged between 

 points when the right line joining them passes through the intervening ground. On 

 account of refraction of the beam of light its trajectory is an upward curve that passes 

 above the right line joining the two points. (For a discussion of the mathematical de- 

 termination of the intervisibility of any two points, see "Manual of Visual Signal- 

 ling," United States Signal Corps, pp. 83-88.) 



In actual practice, however, it is .seldom necessary to operate over ranges of 

 greater than 30 to 40 miles and at these distances any of the types of heliograph em- 

 ployed can be depended upon to give satisfactory service. 



Speed is entirely dependent upon the skill of the operators. It is comparatively 

 easy to attain the ability to send and receive at a slow rate but speed comes only with 

 training and practice. Twelve words per minute may be .sent by skilled operators. In 

 forest-protection work, four to six words per minute may generally be depended upon. 



Section 101 Types of Heliograph 



Although all heliographs operate on the same general principles, there are 

 important differences in the manner in which they cause the revelation and obscura- 

 tion of the flash. Among those which operate by the moving flash method the type 

 employed in the British Army is most commonly seen in Canada. 



Fig. 



HELIOGRAPH WITH SIGHTING VANE. 



British Army type of heliograph, one-mirror 



1 BRITISH ARMY TYPE 



The British Army heliograph consists essentially of a circular mirror mounted 

 on a horizontal axis on which it is free to move. By means of an attached linger 

 key, this mirror may be tipped so as to throw its flash on the distant station when 

 properly adjusted. Between each revelation of the flash, the light is thrown below 



